The present invention relates to a process for the production of alumina from aluminum-containing mineral ore. More particularly, the invention presents a novel process for the production of alumina from bauxite and other similar aluminous mineral ores.
A substantial portion of the world's aluminum needs today are met by the Bayer process for alumina extraction from bauxite. While the Bayer process offers an economically attractive process for the extraction of alumina from bauxite, several important disadvantages are inherent in the Bayer process. First, the Bayer process extracts only the caustic extractable alumina in the bauxite. The alumina associated with silicate minerals present in the bauxite is generally not extracted. Further, between 5 to 10% of the extractable alumina cannot be recovered due to inefficiencies inherent in the process. Second, the caustic soda also reacts with silica minerals present in the bauxite. The extent of this "chemical caustic loss" is dependent upon the amount of caustic reactive silica minerals present in the bauxite. Third, after the alumina extraction, the remaining residue is highly caustic and the slimy nature of this residue or "mud" poses problems in washing the residue. Also, any sodium aluminum silicates present in the residue hydrolyze in the wash water releasing alkali. Further, the disposal of the slimy, caustic residue presents significant environmental concerns to the alumina industry. Fourth, the Bayer process alumina product generally contains various inorganic impurities, such as iron oxide, silica and titanium oxide. The reduction or elimination of these inorganic product impurities, which are related to the basic chemistry of the Bayer process, is considerably expensive. Fifth, the process efficiency is lowered by impurities, such as silica and sodium carbonate, which accumulate in the caustic liquor which is recirculated to the initial step of bauxite digestion in the Bayer process. Controlling impurities present in the recirculated caustic liquor significantly affects the overall production and capital costs associated with the Bayer process.
Processes for producing dawsonite from alumina trihydrate are known in the prior art. For example, U.S. Pat. No. 4,221,771 to van der Heem discloses a process for the production of dawsonite. In this process, alumina trihydrate, which is preferably gibbsite, is reacted with sodium bicarbonate in a sodium carbonate solution. The concentration of the alumina trihydrate is not greater than 86 grams per liter of the sodium carbonate solution. A sufficient amount of sodium bicarbonate is added to react with all of the alumina trihydrate, but the sodium bicarbonate concentration is less than 150 grams per liter, preferably between 52 and 82 grams per liter. The concentration of the sodium carbonate is from 150 to 300 grams per liter. The reaction time is from 30 minutes to 4 hours, and the reaction temperature is between 100.degree. and 200.degree. C. The reaction pressure required is higher than atmospheric pressure, preferably below 18 kilograms per cm.sup.2, and more preferably between 8.7 and 9.5 kilograms per cm.sup.2.
British Patent Specification No. 1,570,261, published June 25, 1980 discloses a process whereby basic aluminum sodium carbonate is produced by the reaction of aluminum hydroxide with an aqueous sodium bicarbonate solution. This patent specification mentions that the reaction temperature is from 160.degree. to 240.degree. C. and the reaction pressure is from 5 to 50 atmospheres. The aluminum hydroxide is in the form of a suspension in the sodium bicarbonate solution, with the suspension being agitated by stirring. Further, the reaction temperature to be employed from the above-mentioned temperature range is chosen in relation to the length desired for the resulting crystal fibers. The patent specification notes that the starting material, aluminum trihydroxide, is a pure material.
U.S. Department of Energy Report LETC/RI-80/8 entitled "Thermal Behavior of Dawsonite" discusses the thermal decomposition of dawsonite. The report indicates that synthetic dawsonite was used and the temperature range under consideration was from 250.degree. C. to 800.degree. C. The report indicates that heating the dawsonite from 350.degree. to 550.degree. C. decomposes the dawsonite to sodium carbonate and rho-alumina, as well as producing carbon dioxide and water. The report further indicates that heating the sodium carbonate and rho-alumina at reaction temperatures of 600.degree. C. and higher produces an incomplete solid state reaction whereby the sodium carbonate and rho-alumina react to form crystalline sodium aluminate, as well as carbon dioxide.
U.S. Pat. No. 3,501,264 to Pilato et al. discloses a process whereby an aluminum salt is reacted with an alkali metal carbonate to produce a basic alkali aluminum carbonate. In the process disclosed in this patent, an aqueous solution of an alkali metal carbonate and an aqueous solution of an aluminum salt are prepared. Prior to admixing these two solutions, the patent discloses an important feature whereby a small amount of sodium gluconate is added to either the aluminum salt solution or the alkali metal carbonate solution. The two solutions are then mixed together at a temperature of from about 20.degree. to 95.degree. C., and the solutions are constantly stirred and maintained at said temperature range for a period of at least 1 hour to allow formation of a basic alkali aluminum carbonate slurry. The slurry is then cooled and aged for at least 2 hours and then filtered to obtain a basic alkali aluminum carbonate filter cake. The filter cake is then washed and dried.